In radio communication, in particular in a mobile communication, a radio channel set between communication apparatuses which carryout information transmission/reception is likely to fluctuate. In order to carry out information transmission using such a radio channel appropriately, transmission parameters of the radio channel is appropriately changed. The transmission parameters include, for example, a modulation method, a coding rate, a ratio of the number of true information bits with respect to the number of total transmission bits including redundancy bits such as error correction bits (referred to as a ‘Rate-Matching ratio’, hereinafter), or the number of spreading codes used in a CDMA (Code Division Multiple Access) communication.
FIG. 1 shows a configuration example in the prior art of a mobile communication system changing the transmission parameters of the radio channel. The mobile transmission system shown in the figure includes a base station 500 acting as a transmitting apparatus and a mobile station 600 acting as a receiving apparatus. Hereinafter, it is assumed that, as the transmission parameters, a coding rate and a modulation method are applied.
The base station 500 includes an encoder 502, a modulation part 504, a transmission part 506, a circulator 508, an antenna 510, a reception part 512, a demodulation part 514, a decoder 516 and a transmission parameter setting part 518. On the other hand, the mobile station 600 includes an antenna 602, a circulator 604, a reception part 606, a demodulation part 608, a decoder 810, a block error rate deriving part 612, a transmission parameter determination part 614, a transmission baseband part 616 and a transmission part 618.
Change of the transmission parameters of the radio channel from the base station 500 toward the mobile station 600 (referred to as a ‘downlink radio channel’, hereinafter) is carried out in the following procedure. That is, based on the coding rate used as the transmission parameter notified of by the transmission parameter setting part 518, the encoder 502 of the base station 500 encodes input data to be transmitted, and outputs it to the modulation part 504. Based on the modulation method notified of by the transmission parameter setting part 518, the modulation part 504 modulates a baseband signal with the data encoded (referred to as ‘encoded data’, hereafter), and outputs it as a signal to be transmitted to the transmission part 506. The transmission part 506 transmits the signal modulated by the encoded data, to the mobile station 600, via the circulator 508 and the antenna 510.
The reception part 606 in the mobile station 600 receives the signal from the base station 500 via the antenna 602 and the circulator 604, and outputs it to the demodulation part 608. The demodulation part 608 applies the modulation method corresponding to the modulation method applied by the modulation part 504 of the base station 500, demodulates the input signal, and outputs the encoded data to the decoder 610. The decoder 610 applies a decoding method corresponding to an encoding method applied by the encoder 502 of the base station 500, decodes the input encoded data, and outputs the data obtained from the decoding. The block error rate deriving part 612 monitors the decoding of the decoder 610, derives a data error rate per a block which is a predetermined transmission unit (referred to as a ‘block error rate’, hereinafter), and outputs the same to the transmission parameter determination part 614.
Based on the block error rate, the transmission parameter determination part 614 determines the coding rate and the modulation method which are the transmission parameters. Specifically, the transmission parameter determination part 614 determines that a propagation environment of the downlink radio channel is bad in a case where the block error rate is high, that is, the data error rate is high, and, changes the coding rate to a smaller one, or changes the modulation method into one applying larger transmission energy per bit. On the other hand, the transmission parameter determination part 614 determines that the propagation environment of the downlink radio channel is good in a case where the block error rate is low, that is, the data error rate is low, and, changes the coding rate to a larger one, or changes the modulation method into one applying smaller transmission energy per bit. Further, the transmission parameter determination part 614 outputs information concerning the new transmission parameters to the transmission baseband part 616.
The transmission baseband part 616 encodes the information concerning the new transmission parameters, and further, modulates a baseband signal of an uplink direction toward the base station 500 (referred to as an ‘uplink baseband signal’, hereinafter) by the information concerning the new transmission parameters, and outputs the same to the transmission part 618. The transmission part 618 outputs the input signal to the base station 500 via the circulator 604 and the antenna 602.
The reception part 508 of the base station 500 receives the signal from the mobile station 600 via the antenna 510 and the circulator 508, and outputs the same to the demodulation part 514. The demodulator 514 demodulates the input signal, and outputs the encoded data to the decoder 516. The decoder 516 decodes the input encoded data, and outputs the information concerning the transmission parameters obtained from the decoding, to the transmission parameter setting part 518.
The transmission parameter setting part 518 recognizes the new coding rate and modulation method based on the input information concerning the transmission parameters. Further, the transmission parameter setting part 518 notifies the encoder 502 of the coding rate, and notifies the modulation part 504 of the modulation method. In data transmission carried out after that from the base station 500 toward the mobile station 600, the encoder 502 applies the new coding rate, and the modulation part 504 applies the new modulation method.
As to the prior arts relating to the above-described transmission parameters, see the following patent documents 1 through 4, for example:
(Patent Document 1)    Japanese Laid-open Patent Application No. 2001-238256;
(Patent Document 2)    Japanese Laid-open Patent Application No. 2001-339458;
(Patent Document 3)    Japanese Laid-open Patent Application No. 2002-84578; and
(Patent Document 4)    Japanese Laid-open Patent Application No. 11-88940.
However, in the above-described method of changing the transmission parameters, a time is required for deriving the block error rate by means of the block error rate deriving part 612. Thereby, the base station 500 cannot change the transmission parameters rapidly in response to a fluctuation of the propagation environment of the downlink radio channel.